1. Field of the Invention
This invention relates to hot isostatic pressing (hereinafter referred to simply as "HIP" for brevity) treatment for sintering or densifying green compacts of ceramics, metal powder or the like.
2. Description of the Prior Art
The technology of the HIP which utilizes an inert gas under a high temperature condition for isostatic compression of a work item or workpiece has been attracting the attention of many concerns as an excellent method for producing sinters of high density from ceramic material, metal power or a mixture thereof, or for crushing residual voids in an ultrahard alloy or for diffusive bonding of metallic materials.
The products which are shaped or sintered by this method have a number of advantages as follows.
(a) A high degree of densification can be attained at a lower temperature as compared with the conventional sintering method, and it therefore becomes possible to obtain fine structure, preventing coarsening of crystal grains due to excessive growth;
(b) A density close to a theoretical value and a uniform structure are obtained in almost any kind of material;
(c) Powder spherical particles which are not suitable for die-molding can be consolidated to a sufficiently high density;
(d) Mechanical and physical properties of powder can be improved;
(e) The fine structure can contribute to the improvement of the properties of, for example, high speed steel tools;
(f) The size of the products is not limited by the press capacity as in the ordinary die molding press, so that it becomes possible to produce larger articles;
(g) A toxic and unstable material can be processed with a minimal affect on health;
(h) There can be produced various composite materials of ceramics, metal powder or the like; and
(i) Material cost can be reduced by an improved yield and a reduction of defective products.
In addition to molding and sintering of powdery materials, the HIP treatment, which is capable of removing internal flaws of an object in a high-temperature and high-pressure atmosphere to increase its toughness and deflective strength, can be utilized for the treatment of sintered tool materials at high temperature and pressure, or for bonding turbine blades to a turbine body to form an extremely strong bond therebetween by diffusive bonding in a high pressure and temperature gas atmosphere.
The HIP treatment which is conducted in a high temperature and pressure atmosphere requires a costly HIP apparatus with a special construction, and usually takes a long cycle time for temperature elevation, pressurization, temperature drop and pressure relief, so that the reduction of the cycle time has been a great technical problem for the enhancement of the efficiency of the HIP operation. In order to solve this problem, there have thus far been made various attempts by using a preheating furnace for raising the temperature of a work item beforehand thereby restricting the operation in the HIP apparatus to pressurization and a certain extent of heating for the purpose of shortening the time period over which the HIP apparatus is occupied by a work item in each cycle of operation, namely, for efficient use of the HIP apparatus. A typical example is found in the apparatus described in British Pat. No. 1,291,459, which is as a matter of fact capable of shortening the cycle time, but has inherent drawbacks in that it entails a large equipment cost for the provision of a preheating furnace in addition to an ordinary HIP apparatus and that it involves an extremely large heat loss while transferring a preheated work item in the atmosphere. Moreover, it has a more detrimental drawback in that the inner wall surfaces in the lower portion of the high pressure cylinder is overheated by the radiant heat from the work item when inserting a preheated work item into the high pressure chamber, damaging the lower seal ring by the overheated inner wall of the high pressure cylinder. However, the safety characteristic of this sort of apparatus should be severely sought often, and the reduction of the cycle time should not be contemplated at the sacrifice of safety. In this connection, applicants disclosed in their copending application, Japanese Laid-Open Patent Application No. 51-124,610, a HIP system of high safety which can perform the HIP operation in a shortened cycle time without imposing adverse effects on the high pressure cylinder or other component parts of the system. More particularly, this copending application is directed to a HIP system wherein a high pressure chamber which is constituted by a high pressure cylinder and upper and lower plugs for sealing the openings at the upper and lower ends of the cylinder is provided with a heater within internal and external heat insulating walls, and a work item is placed on the lower plug to undergo the sintering or bonding treatment in a high pressure and temperature atmosphere, characterized in that the lower plug, heater and external heat insulating wall are integrally detachable from the high pressure cylinder, permitting opening and closing of the treating chamber defined by the external heat insulating wall and lower plug, providing seal means for the treating chamber and a gas passage in the lower plug to communicate the inner and outer sides of the treating chamber with an outside portion of the lower plug. According to this arrangement, it becomes possible to shorten the cycle time by preheating the work item and at the same time to limit the heat radiation from the work item to a minimum, coupled with the prevention of the overheating of the inner wall surfaces of the high pressure cylinder by heat radiation which would shorten the service life of the cylinder, thus ensuring higher security and safety of operation. Further, it is possible to carry out the preheating in a vacuum or a particular inert gas atmosphere, so that there can be employed for the heater or the external heat insulating wall a material which is susceptible to oxidation at high temperatures.
The heater generally employs a heating element of Fe-Al-Cr, molybdenum or graphite, of which Fe-Al-Cr has the highest resistance to oxidation at high temperatures, and is generally accepted as being usable in open air although it retains stability only up to 1100.degree. C. at most. On the other hand, molybdenum- or graphite-base materials which show stability over 1100.degree. C. can be exposed to the atmosphere only in the temperature range of 200.degree.-300.degree. C. as they undergo oxidation to a considerable degree at high temperatures. Therefore, after a HIP treatment in a high pressure inert gas atmosphere at a high temperature of one thousand and several hundreds degrees centigrade, the pressure can be lowered in a relatively short time period but it takes a long time to lower the temperature below 300.degree. C. Consequently, the long time period over which the work item has to be retained in the HIP apparatus for cooling has been a detrimental obstacle to the efficient use of the HIP system. For example, the typical schedule of the time lengths which are required for the respective steps of the conventional HIP process is as follows.
______________________________________ Time Lengths Steps hr. min. ______________________________________ Loading 0. 10 Suctioning .multidot. gas replacement 1. 00 Pressurization .multidot. Heating 3. 00 Retention 2. 00 Cooling-off 8. 00 Press .multidot. relief .multidot. gas recovery 1. 00 Ejection 10 Total 15. 20 ______________________________________
Reduction of the cycle time which is attained by the preheating is restricted to reduction to about 1 hour and 40 minutes for the time period of pressurization and heating which otherwise takes about 3 hours, a reduction as small as 8.7% of the cycle time, and the cooling time period which takes the major proportion of the cycle time has nothing to do with the preheating and still remains as a serious cause of the long cycle time of the HIP operation.